Earth Sciences cover a wide range of scientific disciplines that conduct research on the detailed characterization of geological materials. Scanning electron microscopy is a vital tool in this field and provides high resolving power as well as several analytical capabilities. Secondary electron imaging is essential in micropaleontology where topology is so important while backscattered electron imaging and analytical capabilities are more relevant in mineralogy, petrology and economic geology. The TESCAN portfolio offers dedicated instruments which are capable of satisfying the many demands of specific and general geological applications.
The study of mineralogical and petrographic samples using scanning electron microscopy is now routine in geological research. Similar to optical microscopy but with a much higher resolution, electron microscopy reveals detailed textural relationships between the mineral grains. Its analytical capability has a much wider potential because the interaction between the electron beam and solid matter create several very useful output emissions. Among the most important emissions are backscattered electrons (BSE), secondary electrons (SE), characteristic X-rays and light photons.
- The intensity of the Backscattered Electrons is directly proportional to the average atomic number of the observed phases and is used for imaging, distinguishing individual mineral grains and identifying zones of discrete phases. The contrast resolution can discriminate differences as low as about 0.1 atomic number. The backscattered electron signal enables the user to track zonation in mineral phases and to find optimal analysis points. Similarly, it can be used to visualize and locate specific phases containing heavy elements. This is especially important in searching for very rare, but very valuable, phases that generally form small grains e.g. gold and platinum group minerals. Differences in the BSE intensity can also be used to identify variable orientation of individual crystals of aggregates.
- Secondary Electrons are typically used to observe the morphology of three dimensional samples. They are formed closer to the surface than BSE, have high spatial resolution, a large depth of field and they are less sensitive to differences in the atomic number of the material.
- Characteristic X-rays are the most important output of the electron beam – solid matter interaction as they are used to identify the phases based on their chemical composition. This capability can be used for interactive qualitative assessment but most importantly for quantitative analysis. Special third party detectors compatible with the whole portfolio of TESCAN microscopes are used for this purpose.
- Light Photons are also a useful product of the electron beam – sample interaction. This phenomenon is known as cathodoluminescence(CL) and produces many different luminescent phases among minerals. Many mineral species differ by the colour of the light emission. The colour CL can be used to distinguish different common rock-forming minerals e.g. feldspars or carbonates. CL is also very sensitive to differences in trace element composition and structural order of some minerals. These effects can be observed using panchromatic (black and white) or colour CL detectors. TESCAN manufactures panchromatic detectors and a four channel combined panchromatic and colour detector – Rainbow CL. A combined solution for simultaneous acquisition of BSE and CL images is also available.
Palaeontologists typically use secondary electron (SE) observation to reveal the morphology of fossils. Secondary electrons are formed very close to the sample surface, have a high spatial resolution and reveal very fine details of the sample surface. They also have a larger depth of focus than light microscopy. Electron microscopes are capable of resolving details at the sub nanometre range compared to hundreds of nanometres in optical systems. Higher depth of focus is needed for imaging large three dimensional complex shaped fossils and fossil remnants.
- Electron microscopy is typically used in palaeontology for the identification and taxonomy on microfossils. A conductive coating is usually applied to discharge the electron beam current.
- Microfossils are generally smaller than a few millimetres. The most common organisms are algae, protozoa, and crustaceans. Large populations of microorganisms occurred over a very wide geographical area. Their wide distribution and relative sensitivity to environmental changes have made microfossils into so-called index fossils that are used in the dating of sedimentary sequences.
- Macrofossils can also be studied because of the high depth of focus. Similarly, the bones of small vertebrates can also be studied to determine the causes of different surface marks.
- Specimens which cannot be coated by a conductive layer can be observed under the low vacuum conditions preventing the accumulation of charge.
- A special SE detector was developed for this purpose – LVSTD (Low Vacuum Secondary Electron TESCAN Detector).